1. Field of the Invention
This invention pertains to the preparation of aromatic di- and/or polyisocyanates by thermally cleaving polyurethanes which are prepared by reacting aromatic di-and/or polyamines with O-alkyl carbamates, preferably in the presence of an alcohol and urea.
2. Description of the Prior Art
It is known that aromatic di- and/or polyisocyanates can be prepared by reacting aromatic di- and/or polyamines with phosgene to form the corresponding carbamic acid chloride which can be thermally cleaved into the corresponding di- and/or polyisocyanate. There are several problems with this process: the formation of hydrogen chloride, the toxicity of phosgene, the corrosiveness of the reaction mixtures, and the instability of the solvent. The process is expensive and is difficult to undertake safely. Consequently, there is great interest in producing aromatic di- and polyisocyanates without using chlorine and phosgene.
One method of doing this is by the reaction of carbon dioxide and organic nitro compounds in the presence of noble metals as catalysts at increased temperatures and pressures. See British Pat. No. 1,025,436. Other suitable catalysts are complexes or mixtures of hetero-aromatic compounds and at least one noble metal halide. See U.S. Pat No. 3,576,835 (German Published Application No. 1,815,517). Also, compounds having the general formula PdL(CO)X.sub.2 in which L stands for a Lewis acid and X represents a halogen atom are suitable catalysts. See U.S. Pat. Nos. 3,654,279 and 3,781,321. The problems with these methods are that the catalysts are costly, the reaction conditions must be carefully monitored, and the yields are unsatisfactory.
In order to eliminate these drawbacks, it has been suggested that polyisocyanates be prepared by converting aromatic amines and/or nitro compounds into urethanes which can then be thermally cleaved into the corresponding polyisocyanates. Many attempts have been made to prepare urethanes which can be converted to polyisocyanates. German Published Application No. 2,160,111 (U.S. Pat. No. 3,763,217) and German Published Application No. 2,716,340 (U.S. Pat No. 4,100,351) describe processes for the manufacture of arylurethanes from arylamines such as toluene diamine and/or N-arylamides and dimethyl carbonate in the presence of a Lewis acid. However, dimethyl carbonate must be produced from phosgene and methanol or, according to more recent suggestions, from carbon monoxide and methanol by means of a technically difficult to implement co-oxidation process. However, the co-oxidation process is expensive when compared to the process using phosgene and there is a rather low rate of reaction. Also, N-alkyl-arylamines are formed during the reaction.
German Published Application No. 1,568,044 (U.S. Pat. No. 3,467,694) describes a process by which urethanes can be produced by reacting organic nitro compounds, carbon monoxide, and hydroxyl-containing compounds in the presence of a catalyst which consists of a noble metal and a Lewis acid under essentially anhydrous conditions. The reaction takes place in the absence of hydrogen at increased pressure and at temperatures above 150.degree. C. German Published Application No. 2,343,826 (U.S. Pat. No. 3,895,054) also describes a process by which urethanes are produced from hydroxyl-group containing compounds, carbon monoxide, and nitro-, nitroso-, azo- and azoxy-group containing compounds in the presence of sulfur, selenium, a sulfur and/or selenium compound, and at least one base and/or water. German Published Application No. 2,623,694 (U.S. Pat. No. 4,080,365) describes the manufacture of aromatic urethanes from the above-referenced raw materials in the presence of selenium-containing catalyst systems as well as specific aromatic amino and urea compounds.
However, these processes have considerable drawbacks. They utilize toxic carbon monoxide as a raw material and catalysts which are toxic, or form toxic compounds during the reaction, such as selenium and hydrogen sulfide, or catalysts which are very expensive and difficult to recycle, such as palladium, and they require high technical expenditures with costly safety measures. In addition to this, they can not be used for the synthesis of polyurethanes of the polymethylene polyphenylene urethane type, the potential precursors of one of the technically most important polyisocyanates, crude MDI.
It is also known that phenyl urethane, which is advantageously accessable and which is produced from nitro benzene, carbon monoxide and alcohol, can, according to German Pat. No. 1,042,891 and German Published Application No. 2,832,379, be processed in two stages by condensation with formaldehyde in the presence of large quantities of a strong acid to form methylene-bis-phenyl urethanes and polymethylene polyphenyl urethane (crude MDU). These products can be thermally cleaved to form crude MDI. However, this process is technically not sufficiently flexible and is complicated. It is not sufficiently flexible because of the limited specific adjustment of isomer ratios in the product. The cleaved product contains amounts of 2,4- and 2,2-methylene-bis-phenyl isocyanates which are disadvantageous for important areas of application.
Not only are the processes described for the preparation of urethanes, which can be thermally cleaved to form polyisocyanates, subject to many drawbacks, but the cleaving processes disclosed in the prior art also have limitations. The thermal cleaving of the polyisocyanates is done either in the gas phase or in the liquid phase. Various undesirable secondary reactions simultaneously take place during the thermal cleaving. These include, for instance, the decarboxylation reaction of the urethanes which may be accompanied by the formation of primary and secondary amines as well as olefins, reactions between the resultant isocyanate and urethane to allophanates and/or amine to ureas, and polymerization of the isocyanates to isocyanurates.
In German Published Application No. 1,944,719 (British Pat. No. 1,247,451), the cleaving of urethanes in the vapor phase is carried out at temperatures from 400.degree. C. to 600.degree. C. in the presence of a Lewis acid catalyst with the isocyanate and the alcohols being separated by fractional condensation. The vapor phase in this case is defined in such a manner that the product mixture, possibly also including the solvent, is present in the vapor phase after the cleaving regardless of whether the urethanes to be cleaved are added in the gaseous, liquid, or solid form. Toluene diisocyanate, for instance, is obtained by means of the pyrolysis of toluene-2,4-diethylurethane in the presence of iron-(III)-chloride. Drawbacks of the reaction include lower yields combined with considerable quantities of a polymeric byproduct, decomposition of the catalyst and corrosion of the reaction vessel. German Published Application No. 2,410,505 (U.S. Pat. No. 3,870,739) describes a process by which the urethane is cleaved without catalysts at a temperature of 350.degree. C. to 550.degree. C. and pressures of less than the (m+1 ) times the vapor pressure of the isocyanate product in a catalyst-free pyrolysis zone within 15 seconds. Among others, a drawback of this process is that a large amount of heat required for the endothermic cleaving must be transported to the powdery urethane within a very short period of time. Moreover, a solid polymer, which is produced as byproduct, and its separation make the implementation of a continuous process difficult.
The thermal cleaving of urethanes in the liquid phase is described, for instance, in German Application No. 2,421,503 (U.S. Pat. No. 3,962,302) and German Application No. 2,530,001 (U.S. Pat. No. 3,919,280). According to German Application No. 2,421,503, the urethanes are dissolved in an inert solvent such as alkylbenzene, linear and cyclic hydrocarbons, and/or phthalates, and are cleaved under normal or excess pressures at temperatures of 175.degree. C. to 350.degree. C. The resultant isocyanate and alcohol are isolated and separated with the aid of the solvent as carrier and/or by using an inert gas as carrier. According to German Application No. 2,530,001, higher molecular, possibly substituted, aliphatic, cycloaliphatic, or aromatic hydrocarbons, ethers, esters, or ketones are used as the reaction medium. Only distillation is mentioned for separating the cleaving product with isocyanate, alcohol and carrier materials being distilled overhead whereas the reaction medium remains as bottom fraction.
According to German Published Application No. 2,635,490, urethanes for the manufacture of aromatic isocyanates are brought into contact with a solution of at least one metal ion such as ions of copper, zinc, aluminum, tin, titanium, vanadium, iron, cobalt, and nickel as catalysts. The catalyst is dissolved in a solvent having a boiling point of 200.degree. C. in a metal concentration of at least 0.001 percent by weight relative to the solvent, at temperatures of 150.degree. C. to 350.degree. C. under reduced pressure. The resultant cleaved products are separated by fractional condensation. In accordance with the above-referenced methods, urethanes, dependent upon their structure, can be transformed into isocyanates with yields which are very good in some cases. The manufacture of crude MDI is not described by example in these patents. Differing from the isocyanates which are listed as examples, crude MDI is not completely distillable with the aid of solvents as carriers. Therefore, it can not be isolated as described from catalysts, solvents, unreacted raw materials, and byproducts.